604 HARMON LEWIS 



SECTION I. INTRODUCTION 

 GENERAL 



According to the present general conception the theory of 

 isostasy consists of two main postulates, first that the elevated 

 portions of the earth are deficient in density, and second that the 

 material of the earth is comparatively weak. It is generally ac- 

 cepted that these two postulates are inseparable, for it is argued 

 on the one hand that, if the elevated portions are deficient in 

 density, readjustment involving deformation and failure must have 

 taken place in order to compensate for the large mass of material 

 eroded from the lands and deposited in the sea ; and it is contended 

 on the other hand that, if the earth is weak, it could not support 

 the mountains and continents unless they are compensated by a 

 defect of density below. In accordance with these two main postu- 

 lates it is conceived that the dominant type of earth deformation 

 consists in vertical movements between various segments accom- 

 panied by lateral flowage of rock beneath and possibly crumpling 

 of the rock in the border zones. This conception is not only applied 

 to the major earth segments, the continents and oceans, but to 

 the smaller units of the continents as well. 



The theory of isostasy is a decided contrast to the alternative 

 conception that the earth is strong enough to support the continents 

 and mountains even though there are no compensating density 

 differences, that changes of weight at the surface do not produce 

 vertical movements of the segments in a weaker substratum, and 

 that the dominant type of deformation is folding and upwarping 

 due to lateral compression. 



The theory of isostasy if correct would be of fundamental impor- 

 tance to the geologist in interpreting earth movements. 



Previous to Hayford's geodetic investigation the conceptions 

 of isostasy were largely speculative. After a comprehensive study 

 of the deflections of the plumb bob carried out by the United 

 States Coast and Geodetic Survey under Hayford's direction, 1 



1 The most complete statement of Hayford's work which has been published is 

 contained in his two reports issued in 1909 and 1910 by the U.S.C. and G.S. and 

 entitled, The Figure of the Earth and Isostasy from Measurements in the United States 

 and Supplementary Investigation in iqoq of the Figure of the Earth and Isostasy. 



